While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, the commonly used light exposure technology is approaching the essential limit of resolution determined by the light source wavelength. For the light exposure using g-line (436 nm) or i-line (365 nm), a pattern rule of about 0.5 μm is thought to be the limit. The LSI fabricated using such light exposure has a maximum degree of integration corresponding to 16 M-bit dynamic random access memory (DRAM). However, the laboratory fabrication of LSI already reached this stage, and the development of a further micropatterning technology is in urgent demand.
One means for reducing the pattern size is to reduce the wavelength of exposure light used in forming a resist pattern. For the mass production process of 256 M-bit DRAM (processing size up to 0.25 μm), it is now under intensive consideration to replace i-line (365 nm) as the exposure light source by KrF excimer laser light of a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 1 G or more requiring a finer patterning technology (processing size up to 0.2 μm), a shorter wavelength light source is required, and in particular, photolithography using ArF excimer laser light (193 nm) is now under investigation.
Since H. Ito, G. C. Willson et al of IBM proposed a chemically amplified positive resist composition comprising a resin in the form of polyhydroxystyrene having hydroxyl groups blocked with tert-butoxycarbonyloxy (t-BOC) groups, that is, poly(4-t-butoxycarbonyloxystyrene) (PBOCST) and a photoacid generator in the form of an onium salt, a number of resist compositions having a high sensitivity and resolution have been developed. These chemically amplified positive resist compositions all have a high sensitivity and resolution, but are difficult to form fine patterns with a high aspect ratio because of the low mechanical strength of the patterns.
A number of chemically amplified positive resist compositions using the above-mentioned polyhydroxystyrene as the base resin and having sensitivity to deep-UV, electron beams and x-rays are known in the art. These resist compositions, however, rely on the single-layer resist method although the bi-layer resist method is advantageous in forming a pattern with a high aspect ratio on a uneven substrate. These resist compositions are not yet practically acceptable because of the outstanding problems of substrate topography, light reflection from substrates, and difficulty of forming high-aspect ratio patterns.
As is known in the art, the bi-layer resist method is advantageous in forming a high-aspect ratio pattern on a uneven substrate. It is also known that in order to develop a bi-layer resist film with a common alkaline developer, hydrophilic groups such as hydroxyl and carboxyl groups must be attached to silicone polymers.
Among silicone based chemically amplified positive resist compositions, recently proposed were those compositions comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable and alkali-soluble silicone polymer, in which the part of phenolic hydroxyl group is protected with t-BOC group, in combination with a photoacid generator (see JP-A 6-118651 and SPIE vol. 1925 (1993), 377). Also JP-A 9-110938 discloses a silicone-containing polymer using a silicon-containing acrylic monomer. The silicon-containing polymer of the acrylic pendant type has the drawback that its resistance to dry etching with oxygen plasma is weak as compared with the silsesquioxane polymer. A low silicon content accounts for this weak dry etching resistance. The silicon-containing polymer of the pendant type has drawbacks including developer repellency, poor wetting with developer, weak adhesion to organic film and ease of peeling. To overcome these drawbacks, copolymerization with a (meth)acrylate monomer having an oxygen functional group such as lactone is proposed, but entails the problem that the silicon content is further reduced by introducing the monomer without silicon atom. Then SPIE vol. 3678, pp. 214, 241 and 562 describes a polymer containing a monomer of the trisilane or tetrasilane pendant type having an increased silicon content and a silicon-containing substituent which can be eliminated with acid. However, since compounds having silicon-to-silicon bonds exhibit strong absorption at the wavelength of ArF excimer laser, an increased introduction of such silanes undesirably leads to a lower transmittance. Besides, an attempt of introducing silicon into acid labile groups is reported in SPIE vol. 3678, p. 420. Because of a low acid-catalyzed elimination reactivity, there are drawbacks including low environmental stability and a T-top profile. It is also known that in the case of silicon-containing, acid-eliminatable substituent groups, products (outgases) resulting from elimination have detrimental influence like exposure lens contamination.